I have my AA764 Champ home brew voltages , let me know what you think please

[catnine]

 The reason I bring all of this up is because of the highs I got after raising the bias on the 6V6 . I cannot find anything wrong with the amp other than perhaps the speaker which may be the cause . I have everything posted here http://www.el34world.com/Forum/index.php?topic=11851.0

On the JJ 12ax7

 Pin -1 232 VDC  ( pin 1 and pin 6 seem a bit high)
      -3 1.61 VDC
      -6 230 VDC
      -8 1.74 VDC

 JJ 6V6

    pin -3 plate 354 VDC  ( why is the sreen 10 VDC higher that the plate)
         -4 screen across 470 ohm resister 365 VDC
         -8 cathode 24.6 VDC across 678 ohm bias resister

 B+ is 367 VDC

 Drop across 1K ohm first dropping resister 364 VDC

                  10k ohm second ----------------344 VDC

 Line VAC 116.8 VAC ( this can vary , at night the highest is 119.5 VAC )

 I have checked every resister and all are very close to AA764 spec.

 The cathode bypass cap for 12ax7 pin 1 is 25 uf 25 volt
                                                 pin 6 is 2uf 25 volt

 Here is attached a photo on the dpdt fender slide switch controlling the NFB . What this does is use just the stock 2.7k NFB resister or allow the addition of 100K ohms to the 2.7K NFB , the switch allows me to set the pot of have the stock value .
    

[punkykatt]

Isn`t the AA764 supposed to have two preamp tubes?

[catnine]

Isn`t the AA764 supposed to have two preamp tubes?

 Only if it has trem. A AA764 champ silver face without trem has one 12ax7 , 1/2 the triode is the preamp and the other half is the second gain stage .

[Fresh_Start]

Your voltages are right on spec for the AA764: http://www.el34world.com/charts/Schematics/files/fender/champ_aa764_schem.pdf

Can you swap OT taps just for giggles?

Lowering the B+ to the preamp tube will warm it up a lot.  Try a 47K dropping resistor to the preamp node on the power rail - should drop the voltage by about 100 instead of 20.  150-175 on the plates may be too Tweedy, but the icepick should go away.

As an alternative, jojokeo and tubenit were discussing a "conjunctive filter" just the other day to reduce "chime" in a single-ended amp.  Or you could diddle the tone stack values. Increasing the cathode bypass cap on the power tube to 100uf wouldn't hurt.  Biasing one of the preamp stages differently (like 220K plate resistor & 2.2K cathode resistor) might be another path to happiness. 

Cheers,

Chip

[catnine]

Your voltages are right on spec for the AA764: http://www.el34world.com/charts/Schematics/files/fender/champ_aa764_schem.pdf

Can you swap OT taps just for giggles?

Lowering the B+ to the preamp tube will warm it up a lot.  Try a 47K dropping resistor to the preamp node on the power rail - should drop the voltage by about 100 instead of 20.  150-175 on the plates may be too Tweedy, but the icepick should go away.

As an alternative, jojokeo and tubenit were discussing a "conjunctive filter" just the other day to reduce "chime" in a single-ended amp.  Or you could diddle the tone stack values. Increasing the cathode bypass cap on the power tube to 100uf wouldn't hurt.  Biasing one of the preamp stages differently (like 220K plate resistor & 2.2K cathode resistor) might be another path to happiness. 

Cheers,

Chip

 Wouldn,t lowering the preamp voltage from the second node cause the preamp tubes to break up earlier? or change the way the tone stack functions? I once built a princeton 5F2A and didn't care for the sound all that much so I added a parallel 6V6 , can't say that helped much , on this build I could also remove the 6V6's and plug in one 6L6 since a had an OT with three taps and did not have to change the cathode resister , the 6L6 sounded a bit more clean with less highs. And then it also had a simple tone control which amounted less signal loss.

 What really baffles me is why the 73 fender champ I had did not produce the highs this build does it had the same voltages and tube brands all across the entire amp. Then the 73 had the original tone caps Blue ones and a ceramic disc for the 250pf cap and a weber sig 8 s alnico speaker, that's about the only difference.

[Fresh_Start]

Speaker.

[catnine]

Speaker.

 It maybe the speaker . I don't suppose since the cathode bypass cap being 25uf but 160 volts would matter would it?

[Willabe]

Speaker.

Speaker can make a big difference. What are you using now? I don't think you posted that info.


          Brad         

[Willabe]

I don't suppose since the cathode bypass cap being 25uf but 160 volts would matter would it?

No, the extra voltage rating wont change the treb. response.

What speaker are you using? Do you have access to any others to try? Even a friend or two's?


        Brad       

[catnine]

I don't suppose since the cathode bypass cap being 25uf but 160 volts would matter would it?

No, the extra voltage rating wont change the treb. response.

What speaker are you using? Do you have access to any others to try? Even a friend or two's?


        Brad      

 The only other speaker I have is a Jensen C10R 8 ohm , the one in there now is a weber sig 10s ceramic also 8 ohm running of the OT 8 ohm tap.

[Willabe]

Try it, see what happens?     

Even though there both ceramic 8", who knows.

Have you ever tried the webber you have with any other amp? Maybe it's a bright sounding speaker, maybe it's bad from the factory?


          Brad       

[Willabe]

Why do you have a 678R for the 6V6 bias R instead of the standard 470R value? Was the 6V6 red plating?

On my first build (5E3) I was worried about burning up the OP tubes and put in a bigger KR and it sounded way thin. No bottom end.

So after I passed the fire test I put in the right value KR and it made a huge difference in the tone, bottom end and treb balanced fine.

It does seem a little odd that the screen voltage is 11 volts higher than the plate, should be close to same or a little lower with a 1K dropping R in the power supply. And your plate voltages on the 12AX7 are a little high, by 30v or so.

These 2 things seem to say to me that the amp is not drawing the right current after the 1K in the power supply. Are you sure the supply is wired right?

One other thing that can make an amp sound thin is the layout of not only the components but the wires.

If you have the wrong wire laying too close to another, then you can have big problems. Signal (and power) cancellation being a couple of them, which very well could/will make the tone thin. Could be the FB wiring. I'd take it out and run it stock to see if that's it. The routing of the FB wires/loop/switch could be it. If that's the cause of the thin/bright sound, you will need to figure out how to change the layout on it. Maybe some shielded cable would get it done.  

Can you post a good pic of the build guts? Some of the guys here got eye's like a hawk for things like this.

I'm not clear on if your running 1-6V6 or 2-6V6's? One more thing, what are the transformers you are using?

I think you'll find, and when you find it, you'll be glad you stuck with it.


        Thanks     Brad      

[catnine]

Brad . I started out with the stock 470 ohm cathode bias resister and the 6V6 was running at 19 watts and then put in a 1k that was the deal a few years back when I built this amp. Yet the IPD was 10.25 watts so I kept going down until I was close to 13 watts. I found that when I raised the IPD that is when the amp started with the icepick highs. The amp has one 6V6 and weber tranny's the PT is 125 VAC line and the OT is the same thing as stock AA764 champ uses as far as the specs go as far as I can tell only it has a 4 ohm and 8 ohm speaker tap.

 The amp is dead quiet and all wired proper , trust me I have built more than one amp and this is a real simple amp to build.

 I bought a few of the weber champ PT's but this one had one difference . it has a 300-0-300 secondary and a 330-0-330 secondary . At the time I emailed weber and was told the 300-0-300 secondary was to use if you used a copper cap or diode rect.

 It is wired proper , I am certain of this , I check and check and tripple check before I ever plugged iit in.
 On the PT is does have a 100 mA secondary rather than a 70Ma as a stock fender PT . There is a link to the photo of this amp at the top of this post , well the post I made to begin this post .

 What concerns me is the line VAC was 118.7 VAC when I took the readings I posted which is about what the line voltage was when these amps were built and yes I see my voltages are higher than a fender champ but not more than + or - 20%.

 The amp does not sound thin , it is the fact that the highs are more pronounced , it still has bottom end it is now I have to dial back the treb to cut the highs and before I left all pots full on 10 and got a great tone. Before I posted all the readings I checked the entire circuit out and all resister values and it is wired proper. Trust me I was hoping I would find something wrong. As far as the weber PT and OT who can say how close they are to the stock units yet the voltages do read what they should be off the 5y3 pin 8 . It is quite possible the OT by weber is crap I have no idea yet so far it seems to work proper. I don't have a stock Champ PT or OT to compare them to . My main issue is the higher voltages on the screen and 12ax7 now they were more in line when I used a RCA 6V6 rather than the JJ6V6. As far as the highs go when I back off on the treb I do lose some clarity otherwise I would not care.The NFB pot was after the highs came along , this was an attempt to change things yet it is switched completely to stock by the slide switch.

[catnine]

 I used to mave a 71 and 77 Music master bass amp and learned how to set them up with the very kind help of Jon Bessent . I did recaps and found out how to bias the 6V6's proper and they sounded great. They are low voltage PT's around 290VDC B+ and are called the big brother to the champ even though they were a bit different. My 73 silver faced champ came from Ebay loaded with some torres mode and sounded like a grinding machine , horrid mod but the seller gave me all the parts so i put them back and recapped it and biased it and it sounded great . Back then I made a tall cab that was just like a fender MM bass amp cab  but tall enough to hold two 12's one on top of the other and I could also fit the 73 champ chassis in this cab because the MM bass amp and champ are the same chassis other than drillings and tubes but the same chassis stamped out . The 73 champ made the two 12's parallel to 4 ohms sound fantastic but these days i don't have all sorts of eminence and other speakers to try out and I had perhaps 20 twelves and 5 10" speakers. In fact fopr a time I had the 73 champ chassis in the 71 mm bass amp cab with a weber sig s 10 alnico which was then offered in a 4 ohm and 8 ohm , well I had to back off on the bass control a bit with the ten . here is the tall cab still here sitting.

[catnine]

 I wanted to add . in this tall MM bass amp cab I built . I have a 71 MM bass amp chassis that was doa so I made a 6G2 front end to a 5E3 power section using Allen amps tranny;s and could use 6V6's or 6L6's and with the 6L6's and a GZ-34 rect and two heavy duty eminence 12's it blew to 1994 fender tweed deluxe I had out of the room. I still have that built in the 71 mm bass amp  cab with 6V6's and one 8 ohm 12. Too much amp for here . I was experimenting at the time.

 I was in the building phase bigtime back in 2002 to 2005 . Now I am lucky I have spare 6V6's and 6L6's and 12ax7s and a few 12at7s and one spare speaker. I can't afford this any more so i have to make do with what I have and a box of all sorts of resisters and electrolytic caps and other caps. I see all the builds people do here and all are really , really well done.   

[catnine]

Try it, see what happens?     

Even though there both ceramic 8", who knows.

Have you ever tried the webber you have with any other amp? Maybe it's a bright sounding speaker, maybe it's bad from the factory?


          Brad       

 The weber sites description does say this is a bright woody /reedy sound. The Jensen site on the c10R says about the same but I will try it . if memory serves it is well broken in and I've have the Jensen for some time and I do recall something was lost when I got the weber sig 10 s ceramic yet it's been years so i have to try it and see.

[Willabe]

What concerns me is the line VAC was 118.7 VAC when I took the readings I posted which is about what the line voltage was when these amps were built and yes I see my voltages are higher than a fender champ but not more than + or - 20%.

Hi catnine,

Well sounds like you've been over it with a fine tooth comb.
 
But I still don't understand why the +/- 20% would not include the 6V6 plate voltage too, if it was the PT. (I know you did not say that it was.) If you checked all the R's and their good/right, why would only the screen and plate voltages be higher only -- after -- the 1K screen dropper in the power supply? I don't get it. And again why is the screen voltage 11v higher than the plate after the 1K screen dropper R? You asked the same thing earlier. Why is'nt the current draw/load of the 6V6 screen and 12AX7 plates bringing down their voltages?  


What ever it is, it sounds like you will find it.  


           Thanks,      Brad        

  

    

[catnine]

Brad , you got me. I did install a grid stopper of 1500 ohms on the 6V6 and a screen resister of 470 Ohms but 470 should have perhaps dropped the screen just a tad . AA764 champs don't have either resister but I can't imagine that's the problem .

[Willabe]

Ehh, I ain't got nothin but bill's.   

Those are good to add from what I understand. That's not the problem.

I'm tryin to say I think it's -- after -- the 1K screen droping R in the power supply that somethings not right.

The load/current draw from the 6V6 screen and the 2 plates of the 12_ _ 7 -- should -- cause the screen and 12_ _ 7 plate voltages to go down and -- not -- up.   


          Thanks,      Brad       
 

[catnine]

 I understand exactly what you are saying Brad . I checked EVERYTHING once again . I find not one thing wrong . I even changed the preamp 12ax7 from a JJ to a sovtek and the only thing that happened was the pin 1 was a bit lower than pin 6 but still about 230 VDC . I changed the JJ 6V6 with a EH 6V6 still had the same 11 VDC higher screen . The 1k dropping resister drops 4 VDC from B+ across it and at the screen yet the resister does read 1k ohms . After the 1k comes the 10k and it drops another 21VDC . I checked with a DVOM measured before the 10K to the junction of the two 100K 12AX7 plate resisters  and got 112K total and the same at the socket pins.

 All the resisters check out fine as do all the grounds at every point on the entire circuit . I drop 109 VDC across the 100k plate resisters to the plates on the 12ax7.

 For the heck of it I checked across the B+ on the rect tube and then there is a fuse and a 47 ohm 5 watt resister to hold back the surge to the 1st 47uf filter cap before the standby sw . There I get a 1 VDC drop . it's the same across the fuse , the drop is only after the resister just to see if the fuse was an issue .

[Fresh_Start]

With a 1K resistor between the first node on the power rail (OT) and the screen grid node, it's normal for the screen grid voltage to be a little bit higher.

Chip

[catnine]

With a 1K resistor between the first node on the power rail (OT) and the screen grid node, it's normal for the screen grid voltage to be a little bit higher.

Chip

 thank you Chip

[catnine]

The reason I bring all of this up is because of the highs I got after raising the bias on the 6V6 . I cannot find anything wrong with the amp other than perhaps the speaker which may be the cause . I have everything posted here http://www.el34world.com/Forum/index.php?topic=11851.0

On the JJ 12ax7

 Pin -1 232 VDC  ( pin 1 and pin 6 seem a bit high)
      -3 1.61 VDC
      -6 230 VDC
      -8 1.74 VDC

 JJ 6V6

    pin -3 plate 354 VDC  ( why is the sreen 10 VDC higher that the plate)
         -4 screen across 470 ohm resister 365 VDC
         -8 cathode 24.6 VDC across 678 ohm bias resister

 B+ is 367 VDC

 Drop across 1K ohm first dropping resister 364 VDC

                  10k ohm second ----------------344 VDC

 Line VAC 116.8 VAC ( this can vary , at night the highest is 119.5 VAC )

 I have checked every resister and all are very close to AA764 spec.

 The cathode bypass cap for 12ax7 pin 1 is 25 uf 25 volt
                                                 pin 6 is 2uf 25 volt

 Here is attached a photo on the dpdt fender slide switch controlling the NFB . What this does is use just the stock 2.7k NFB resister or allow the addition of 100K ohms to the 2.7K NFB , the switch allows me to set the pot of have the stock value .
    

 I still do not understand why the plates are so high on the 12ax7s when all the resisters are correct and everything is wired proper.

[Fresh_Start]

I still think the biggest problem is the speaker, or possibly tube choice.  But probably speaker & cab.

I still do not understand why the plates are so high on the 12ax7s when all the resisters are correct and everything is wired proper.

Rather than worrying about why, let's just figure out how to "fix" it.  2ma of current might seem a little bit low for a 12AX7 but not unreasonable with 100K plate, 1.5K cathode, and 344 VDC high voltage supply for the preamp.  However, 2ma is what you have (V/R=I so 20/10,000=2ma).  FWIW I usually assume 1.5ma per 12AX7 triode for starters but 100K/1.5K rarely draws that much current.

Note that your B+ for the preamp is only 14 volts off spec - well within the +/- 20% allowance.  For that matter, the 12AX7 plate voltages are only 15% above what's shown on the schematic and within spec.

If you replace the 10K dropping resistor on the power rail with a 27K resistor, you should get about 310 VDC supply for the 12AX7s (.002ma * 27,000 = 54 volts).  That should drop your 12AX7 plate voltage(s) down below 200 volts and may give you a "rounder", mellower tone from the preamp.

You also could bias the first triode a bit hotter/differently.  Either a 1K cathode resistor or a 168K to 200K plate resistor might be fun to try.  Depends on how clean you want the amp to sound and how much headroom you want.

Another area to explore is the tone stack.  Maybe a 330p treble cap would help.  Tack a 100p cap in parallel with the 220p that's there now and listen.  Maybe you'd like two .022uf mid/bass caps like a Marshall instead of Fender's .1uf bass & .047 mid.

For me, building an amp is like cooking.  I start with a recipe, sure.  I taste throughout the process and adjust proportions as I go along.  Maybe I decide to leave something out (never the garlic though), maybe I throw something else in.  Some things I measure.  Most things I just eyeball in terms of quantity - I'm more interested in relative proportions.  Maybe I make a note to do X differently next time.  Everything from your finger tips to the speaker cone is part of the instrument, including guitar strings and coupling caps.  That's a LOT of possible variables in combination.  Too many variables to know exactly how it's going to turn out.

The AA764 is a classic circuit.  If you have an NOS 12AX7, a Tung Sol 6V6 and a real 5Y3-GT, that's a start.  Pine cab same dimensions plus a Weber 8A100-T speaker and you're on your way to an amp that should sound like a BF Champ would have in 1960 whatever. 

The OT is a big part of any amp's sound too.  Have you told us what the OT is?  The voltage drop across the OT secondary is a little bit higher than "spec" which might indicate a higher secondary winding resistance.  Whatever it is, the secondary winding resistance is greater than 1K.  We know that because the voltage drop across the OT is greater than the voltage drop across the 1K dropping resistor going to the screen grid supply.

The point of all this is that you've built the amp to vintage specs (with the possible exception of the speaker/cab/OT).  Voltages are within 20% of spec, so nothing's drastically wrong.  You want it to sound different.  Change it to taste the way you like it.

Good luck,

Chip

[catnine]

 Chip , the OT is a weber replacement for a champ with a 7k ohm primary but does not state what the secondary is . All I know is it is the same size as a stock Champ OT but has a 4 and 8 ohm secondary tap , weber also has one with just a 4 ohm tap but they are rated and are the same size which really does not tell you much. They are supposed to be built to fender specs so the site says yet is it or any replacement OT just like the original , I sort of doubt that . I got it mainly because it had the 8 ohm tap whcih is the most common speaker used these days. The PT is also weber a stock champ replacement . As far as tubes the 5Y3 is an NOS westinghouse , the 6V6 I used an old but good RCA 6V6GT and have a JJ 6V6 in it now and tried a EH 6V6 . The RCA does drop 6 VDC off the screen the plate and cathode have the same readings as the JJ . I have tried NOS 12ax7s and sovtek and JJ , the JJ cuts some highs so i kept it in.

 One thing I thought about was since in order to get the 6V6 to draw no more than 14 watts like many I raised the cathode resister from 470 ohm to 680 ohm so this alone would cause the 6V6 to draw less current which I assume would do the same with the 12ax7 and therefore  raise the VDC all the way down the line.

 If changing the 10K to a 27k would and I do see how , lower the 12ax plate voltage and offer a mellower tone then I can do that . I just want less highs and clean tone . None of the 12ax7's I have change the plate voltage or current , what the NOS 12ax does is really ad more highs , the JJ 12ax is a notable difference in that respect.

 I build this amp to spec as best I could I really did not want to change things , at least that was my goal . I took these reading when my line VAC was 116.8 then later I took them when it was 118.2 all the VDC readings went up by 17 VDC yet the 12ax plate rose only 10 VDC . The 6V6 went from 12.80 watts to 14.2 watts . I have no control over the old apt varying line VAC.

[Fresh_Start]

No problem starting the build as close to spec as possible.  Makes sense to me.  Until you play it and don't hear what you want to hear...

Tube choice sounds OK, but a new Tung Sol 6V6 and/or 12AX7 might help.  I always found JJ 12AX7s too muddy/dark but that may be the solution for your amp.

I missed the detail "2uf on pin 6".  While it won't completely fix your problem, I'd use at least a 4.7uf or 6.8uf cap there:
http://www.freewebs.com/valvewizard2/OtherStuff.html
For perspective, low E string is about 82Hz.

Also, you could try a combination of increasing the 10K dropping resistor and the 1K dropping resistor.  IOW a 20K and a 2.2K.  Keep in mind that the 2.2K probably will introduce a bit more "sag" since when the screen grid draws more current it will pull the voltage down.

Do you want less high end but still clean?  Or is it less high end and more dirt?  Based on your comments about not liking your 5F2-A build, I'm guessing it's less high end but still clean. 

Speaker.  Tubes.  Output transformer.  Plus several of us have suggested circuit changes that might.  A conjunctive filter across the output tube, a grid stopper on the second 12AX7 triode, bigger treble cap in the tone stack, or a small plate resistor bypass cap on the second triode, etc.

I love Weber speakers, but have not read much good about their transformers - especially output transformers.  Please keep in mind this is ALL second hand info, but you might want to try another OT.  I'm sure Hoffman's unit would be a good choice - I do have direct experience with Hoffman's transformers.  It only has a 4 ohm secondary though.  Allen Amps sells really good iron too - I've got a TO8C in a Tweed Princeton (sort of) that would sound great in a Blackface Champ.  It's a bit too "good" for the Tweed IMHO - too much bass and too clear (speaker is a 10A100) - but you'll probably like it for this build.  Hammond's 1760C might be closer to vintage spec but I'm not sure about that.

HTH

Chip

[catnine]

Chip thanks for the suggestions.

 You mentioned raising the rial resisters to 10k and 20K , I assume the 2.2K is the 6V6 screen resister in place of the 470 ohm I have in there now. Also the PT I have in this build has 2 HT secondaries , 330-0-330 and 300-0-300 VDC . I have the and on the 330 then switched it to the 300 and changed the 680 ohm cathode bias resister to a 500 ohm to get the 6V6 back to 13 watts thinking well I'll lower the entire thing voltage wise, simple two wires , man did that kill the bottom end and the clean . Turned it into a sterile 5F2A with a BF tone stack. Mainly  I think because of the low plate voltage on the 6V6. All older fender AA764 champs with stock PT's are running on higher line as you know and are 325-0-325 VDC . I have also read all sorts of opinions on how to bias them at the higher line and have read let them run at 18 watts or drop the B+ using assorted zener diodes and don't go over 500 ohms for the cathode resister or the 6V6 will clip un-evenly. I had champs in the 80's , didn't know about bias and they sounded fine to me , still had I assume stock RCA 6V6's but they do sound better biased no more than 14 watts.

 I don't know ohms law much at all . I can read a wiring schematic. Yet this is from working on cars for 35 years . I can test amps and voltages , figure out how to splice a preamp section of one circuit to the power section of another and leave out the trem circuit and build an adjustable fixed bias , yet tube amps are not quite the same as a cars 12 volt circuit so I will miss a few things as to how they actually work. I am just a long time blues guitar player who decided to build his own guitars and then looked at amps and thought , I can do this , mix in theory and math of an amp , I am lost . I understand it but could not design it.

 I know OT's do make a difference , in the 5F2A I built I used an Allen amps TO11C and a rather large hammond for the PT can't recall the number of that PT , The OT allowed the use of 2 6V6's or one 6L6 and since I decided to build a parallel 6V6 version of the 5F2A and had 8 ohm speakers I chose the TO11C . I could run one 6L6 and not have to rebias . After all of that I would not say there was any sort of big change from the amp I started out with , just  tad more clean . While I was building it I had all of these great expectations . This is not to dispute you suggesting the Allen amp OT would be better than the weber I'm sure it would be.

 I used Allen amp tranny's to build my 6G2/5E3 . TP25 PT and a TO 25 OT which is now the TO26 and that build sounds out standing but then it is a push/pull and all together different than a SE champ . But it started out as a 71 MM bass amp with a bad OT and got his OT and could hear a difference.  

 Basically at this point I do have resisters and caps and other componants I can play with value wise or addition wise but nothing in the way of tranny's and but one 10 speaker and they are just not in the budget any time soon.

 I did find another MM bass amp OT but by then I rebuilt the entire MM bass amp because the PT was acting up . I then decided to build two 6G2's without trem and used the 6V6  MM bass amp OT in one and a weber Princeton 6G2 replacement OT in the other , the amps were the same in every way and sounded the same , the OT's read the same and looked the same . At the time I called weber to find out if the PT he had for the princeton would work in the Music Master bass amp .